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We investigate the effect of the length scale of wetting heterogeneities, close to the length scale of a pore, on capillary pressure saturation (CPS) curves and the United States Bureau of Mines (USBM) and Amott-Harvey (AH) wettability indices. These macroscopic wettability indices are used to describe bulk rock wettability, because the local contact angle (the standard physical measure of wettability) in a sample is difficult to access and might vary within and between pores caused by changes in mineralogy and the surface coverage of organic materials. Our study combines laboratory experiments and full-scale fluid-dynamics simulations using the multiphase stochastic-rotation dynamics (SRDmc) model. Four model systems were created using monodisperse glass beads. The surface properties of the beads were modified so that one-half of the surface area in each system was strongly hydrophilic and the other half was hydrophobic. However, each system had a different length scale of wetting heterogeneity, ranging from a fraction of a bead diameter to two bead diameters. There is excellent agreement between the experimental and simulation results. All systems are classified as intermediate-wet on the basis of their AH and USBM indices. An examination of the capillary pressure curves shows that the opening of the stable hysteresis loop decreases monotonically as the length scale of the wetting heterogeneities is increased. Thus, our results suggest that macroscopic wettability indices could be used as indicators of ultimate recovery, but they are not suited to discriminate between the different flows that occur earlier in a mixed-wettability displacement process.
